The present invention relates generally to the field of electrophotographic imaging techniques, and more specifically, is directed to the use of a foraminated device which is capable of having stored thereon a charge pattern corresponding to the light and dark areas of a graphic original and which device can then be used to selectively transmit and otherwise block the passage of charged particles directed towards its surface. The ions which are permitted to pass through the modulator are then collected on a dielectric material and developed into a visible image. For the purpose of the description of this invention which follows, the term "charged particles" will be deemed to define not only toner particles but also gas ions which are deemed to be charged particles. The term blocked, when used herein, will describe the condition of the charged particle not passing through the particular aperture in the modulator due to being attracted to the modulator or otherwise repelled from the aperture into the stream of particles to be projected into another aperture.
Electrophotographic reproduction techniques for making reproductions of graphic originals using photoconductive media are well known. Such processes call for applying a blanket electrostatic charge to a photoconductive layer, and then exposing it to a pattern of light and shadow created by directing electromagnetic radiation onto such graphic original and then projecting the resulting pattern by means of an optical system onto the light-sensitive photoconductive layer. In the light-struck areas of the layer, the charges are conducted to ground, leaving behind a charge pattern corresponding to the dark or shadow areas of the graphic original. The images are rendered visible by the application of an electroscopic powder which is then fixed directly onto the photoconductive layer, or can be transferred and then fixed on a suitable receiving medium such as plain paper.
This technique of electrophotographic reproduction has been extended in this art to foraminated structures which are formed by applying a photoconductive layer to a wire screen mesh or similar apertured structure. The response of the photoconductive medium in such an apertured structure is the same as is experienced in conventional electrophotographic imaging techniques in that the photoconductive layer can be charged, thereby rendering it sensitive to electromagnetic radiation, and thereafter exposed to a pattern of light and shadow to create an electrostatic charge pattern thereon. Such foraminated structures are known in this art as photoconductive screens, modulators and apertured photoconductive materials. In describing the structures of this invention, the term "modulators" will be used to define the apertured devices capable of accepting an electrostatic charge and responding to a pattern of light and shadow to have recorded thereon an electrostatic charge distribution system which can be utilized in ordinary room light without affecting the continued existence of any charge pattern on the surface thereof.
The photoconductive screens which have been developed heretofore and are known in the prior art fall into three distinct classes:
The first is a two-layered screen construction which is formed by applying a photoconductive layer onto a metallic screen. Such a structure is capable of accepting an electrostatic charge corresponding to a pattern of light and shadow created by electromagnetic radiation directed onto a graphic original. The operation and construction of such a device requires that the projection of ions through the screen occur simultaneously with the projection of the pattern of light and shadow. The simultaneity requirement is occasioned by the inability of such a system to retain or have any "memory" in terms of the charge pattern imparted to the structure.
A second group of photoconductive screens has been adapted for use with charged material particles but not gas ions. Such structures suffer from the deficiency that charged particles accumulate in those areas of the structure which attract the particles. Ultimately, it is required that the screen be cleaned to physically remove the particles in order that the screen may be reused.
The third group of screen-type structures known in the prior art are essentially two-layered structures containing a nominal third insulating layer which is included for the purpose of providing a proper surface resistivity in the circumstance that the photoconductor does not have the proper range of resistivity and thereby improving the charge-carrying capacity of the photoconductive layer. Such screens have been described as having the capability of projecting gas ions through certain areas of the screen while preventing their passage through other areas which attract such ions in accordance with the charge pattern imparted to the photoconductive layer. The time duration of the ion discriminating capabilities of such a photoconductive screen structure is a function of the dark decay properties of the photoconductor. Of necessity, therefore, any electrostatic fields which have been created in the apertures of the photoconductive screen by virtue of the charges accepted by the photoconductive layer are in a continual state of decay.
To more fully appreciate and understand the modulator structures of this invention, it is necessary to discuss the known processes for imaging a photoconductive member which has been overcoated with an insulating layer. These processes employ continuous photoconductive members formed by applying a suitable photoconductor to a conductive layer and then applying a third layer which is a highly insulating, transparent material, over the photoconductor. The application of a blanket electrostatic charge to the insulating layer results in the injection of oppositely poled charges into the conductive layer in the direction of the insulating layer. The injection of such oppositely poled charges, which ultimately are bound at the interface between the photoconductive layer and the insulating layer, occurs by virtue of the rectifying properties of the photoconductive layer. This dipole charge state could also be achieved by applying a blanketing charge simultaneous with flood illumination using a non-rectifying photoconductor. The insulating layer thus has a uniform field applied across its thickness, and any previous charge distribution system (CDS) has been satisfactorily erased for the receiving of a new CDS.
Upon simultaneous exposure to a corona electrode connected to an AC supply source and a projected pattern of light and shadow, the charges on the insulating layer are erased by the corona. The charges which are bound at the interface are leaked to ground by rendering the photoconductive layer conductive in the light-struck areas.
In the dark areas the effect of the AC corona is to connect the top of the insulating layer to the conductive layer, causing some of the charges to be transferred to the metal base and causing oppositely poled charges to be bound at the interface between the photoconductive layer and the metallic layer. In the dark areas, the outer insulating layer and the conductive layer are at an equal potential level. The net charge across all three layers is zero and the ability to control charged particles directed to the modulator does not occur until the member is flood illuminated.
As a final step, the photoconductive member is given a flood exposure of electromagnetic radiation which now causes the photoconductive layer, overall, including the formerly dark portions, to become conductive, causing these charges to leak off to ground, leaving those charges that are bound at the interface between the photoconductive layer and the insulating layer by the remaining charges on the surface of the insulating layer. Hence, a dipole charge system is created across the insulating layer in those areas corresponding to the shadow or dark portions of the graphic original.
In another system for creating a charge pattern on such a three-layered structure, there is applied a blanket electrostatic charge under conditions in which the photoconductive layer does not possess special rectifying properties and hence a charge is produced across the insulating layer and a corresponding, oppositely poled charge between the photoconductive surface and the metal layer. Again, any previous CDS is also erased for the receiving of a new, fresh CDS.
In the next step of the process, the photoconductive layer is exposed to a pattern of light and shadow so that the light-struck areas will cause the migration of charges to the interface between the insulator and the photoconductor. In the dark or shadow areas, the arrangement of the charges remains the same. As the third step, a charge from an AC corona is applied to the photoconductive medium in the dark areas which reduces the charge in the insulating layer in the non-exposed dark portions, but in the light-struck areas, where the charges have migrated to the interface, a fraction of the original charge remains.
As the final step of the process, the photoconductive member is given a blanket illumination which reduces the voltage in the photoconductor, causing the charges to be conducted to ground and produces a net charge of a polarity opposite to that of the DC corona in the light-struck areas of the member.
It is important to note that in the first process described, the developable image occurs in the areas that have not been light exposed, whereas in the second process, it is the light-struck areas that represent the developable image or CDS.
The aforedescribed photoconductive members of the prior art are known to have the capability of retaining the charge pattern that has been created on its insulating surface for extremely long periods of time and the persistence of said charge pattern to be unaffected by electromagnetic radiation, particularly radiation in the optical spectrum.